1. Field of the Invention
The present invention pertains to control of fluid flow. More particularly, the present invention is related to methods and apparatus for control of fluid used to drive and operate a system, or a plurality of systems wherein the systems are interdependent. The methods and apparatus of the present invention find application in the control of the flow of driving fluid through turbines, particularly systems of two or more turbines operating in series.
2. Description of Prior Art
Modern turbines are generally equipped with primary nozzles whose cross sectional areas may be selectively varied to control the flow of the driving fluid through the turbines. For a given drop in pressure of driving fluid across a single turbine, the volume flow of driving fluid and, hence, the energy transferred to the turbine, increase as the nozzle is opened, and the speed of the turbine is increased. The load driven by the turbine also influences its speed, causing the speed to decrease as the load is increased.
When two or more turbines are operated in series, the total drop in driving fluid pressure is distributed, in general, among all such turbines. Thus, a change in the pressure drop across one nozzle, effected by a variation in opening of that nozzle, affects the pressure in the driving fluid supply line in general, and, therefore, the pressure drop across all the other turbine nozzles. Opening the nozzle of one turbine in a series reduces the pressure drop across that turbine, slowing it down, and increases the pressure drop across the other turbines, speeding them up. Thus, the relative speeds of turbines in series are determined by the relative degree by which each turbine's nozzle is open or closed. Thus, in general, for like turbines with equal loads, if all nozzles are open, say, 40%, all turbines run at the same speed. If the nozzle of one such turbine in the series is open 80%, that turbine will be running at approximately half the speed of the turbines with nozzles open 40%. As in the case of a single turbine, varying loads on the individual turbines cause the respective turbine speeds to vary also.
The customary method of adjusting turbine speeds for a series of turbines is to manually preset the relative degree of opening of the several turbine nozzles, then to effect all speed adjustments during turbine operation en-masse, by a single flow signal to all the nozzle actuators. Thus, for example, certain turbines in the series may be preset to operate at one-half the speed of the remaining turbines by manually adjusting their respective nozzles accordingly. Then, during operation, regardless of the actual speeds involved, the first set of turbines will operate at one-half the speed of the second set. This ratio of speeds will be preserved as the flow signal is used to increase or decrease the speeds of all the turbines by decreasing or increasing, respectively, the openings of the turbine nozzles. However, once the nozzle ratio is manually preset, it must be manually adjusted again to accommodate any changed conditions concerning one or another of the turbines of the series.
Another concern in varying the speed of turbines is the critical speed associated with the resonant frequency in a component of the individual turbine. For a given turbine, there may be one or more speeds of operation at which the frequency of rotation of the rotor corresponds to such resonant frequency such that continued operation at that speed may result in severe vibration and ultimate damage to the turbine. Such critical speeds are generally encountered at something less than the range of normal operating speeds. Consequently, it is common practice, in starting up a turbine, to manually bring the turbine up to a speed just under the critical speed, then to quickly accelerate the turbine through the critical speed. Similarly, in slowing down a turbine, the critical speed is approached from above by manual control, and the turbine rapidly decelerated through the critical speed.
It will be appreciated that maneuvering all the turbines coupled in a series through their respective critical speeds, as described hereinbefore, can be a tedious and unreliable operation. Whether in a series of turbines or in the case of a sole turbine, the reliance on manual control to avoid critical turbine speed is generally undependable since a turbine may be inadvertently be operated at its critical speed for a period of time sufficient to result in turbine damage. The problem may be compounded where a given turbine exhibits more than one critical speed.